Solid state light sources such as light emitting diodes (LEDs) generate visible or non-visible light in a specific region of the electromagnetic spectrum depending on the material composition of the LED. When it is desired to construct an LED light source that produces a color different from the output color of the LED, it is known to convert the LED light output having a peak wavelength (“primary light”) to light having a different peak wavelength (“secondary light”) using photoluminescence.
Photoluminescence generally involves absorbing higher energy primary light by a wavelength converting material (“conversion material”) such as a phosphor or mixture of phosphors. This absorption excites the conversion material to a higher energy state. When the conversion material returns to a lower energy state, it emits secondary light, generally of a longer wavelength than the primary light. The peak wavelength of the secondary light can depend on the type of phosphor material. This process may be generally referred to as “wavelength conversion.” An LED combined with a wavelength converting structure that includes a conversion material such as phosphor to produce secondary light may be described as a “phosphor-converted LED” or “wavelength converted LED.”
In a known configuration, an LED die such as a III-nitride die is positioned in a reflector cup package and a volume. To convert primary light to secondary light, a wavelength converting structure (“wavelength converter”) may be provided. The wavelength converter may be integrated in the form of a self-supporting plate, such as a ceramic plate or a single crystal plate. In any case, the wavelength converter may be attached directly to the LED, e.g. by wafer bonding, sintering, gluing, etc. Such a configuration may be understood as “chip level conversion” or “CLC.” Alternatively, the wavelength converter may be positioned remotely from the LED. Such a configuration may be understood as “remote conversion.”
In one particular known configuration, the wavelength converter can be constructed as a thin film, typically deposited on a transparent substrate material such as sapphire. For example, U.S. Patent Publication 2012/0261688 describes a wavelength converting thin film structure having a non-uniform top surface created during deposition of the wavelength converting material on a polished sapphire substrate. In another example, U.S. Patent Publication 2013/0313603 describes an epitaxial wavelength converting thin film structure on a transparent substrate such as Gd3Ga5O12 or Y3Al5O12 whereby the emission wavelength of the conversion material may be tuned by lattice strain engineering instead of by varying the elemental composition.
One potential issue with thin-film wavelength converters is that they are likely to have fewer scattering centers (e.g., grain boundaries) compared to ceramic wavelength converters formed by sintering powdered materials or casting resins containing dispersions of phosphor particles. This means that the thin-film converters are more likely to experience losses due to total internal reflection (TIR). Light extraction may be improved by roughening the surface of the converter, e.g., mechanical abrasion, chemical etch, etc. However, these techniques may produce a generally uncontrolled effect with varying degrees of surface roughening and the thickness of the thin film may limit their application.